A letter from the nuns at Sean Ross Abbey in County Tipperary to adoptive parents, published this week, throws a harsh light on the prevailing attitudes of the clergy to unwed Irish mothers and their offspring, who were often forcibly adopted for cash, records show.

Mary Lawlor was offered for adoption by the nuns at Sean Ross Abbey in the early 1960s. This week she published letters to her adoptive parents from the nuns that underline their disapproving attitudes to the children of unwed and poor mothers.

In the letter the nuns warn Lawlor's adoptive parents not to choose a child of the "wrong class," and they advise them that "the better class girl has to leave here quickly so as not to be detected in her sorrow," implying that her child will be much younger and easier to parent.

In the letter dated July 26, 1961, the sister in charge of the Roscrea institution writes to Lawlor's adoptive parents:

"We had a wonderful reference from your priest and we think you should take a baby over six months, for one reason the baby will be brought up just as you would bring your own child up and a child of two years has been too long in an institution to fall easily into your ways."

The letter continues: "Another thing is the better class girl has to leave here quickly so as not to be detected in her sorrow, so the better class child will be younger... We have a very nice little girl Mary Margaret who is of good background and very intelligent," the nun wrote.

Speaking to the Irish Mail on Sunday, Lawlor added: "They were picking and choosing babies, so the older ones – who would have needed a bit more support – ended up being left there because the nuns were putting people off them."

Lawlor helped organize the first annual memorial at Sean Ross Abbey last week with fellow adoptee Edel Byrne. The ceremony included readings from the letters of those affected by forced adoptions.